Cancer is a disease characterized by changes in the intricate cellular network that normally regulates processes such as proliferation, differentiation and apoptosis. The functions of many cell cycle regulatory proteins are compromised or altered in a substantial proportion of human tumors. An example of such a molecule is the product of the retinoblastoma gene RB, whose loss is the rate-limiting step in the development of retinoblastoma. The encoded pRB protein was initially shown to be a negative regulator of proliferation but more recent studies suggest that pRB also plays a role in the control of differentiation, as well as in apoptotic and checkpoint responses. The pRB protein belongs to a protein family that includes the closely related p107 and p130 proteins, that exhibit both overlapping and differential functions to those of pRB. Relevant to this proposal, pRB and p107 exhibit opposing roles in Ras-mediated transformation. In this setting p107, but not pRB, acts as a tumor suppressor. In fact, maintained pRB expression seems to be required for Ras-mediated transformation, an observation that could explain why tumors that harbor Ras mutations almost invariably retain expression of wild type pRB protein. These novel observations will be extended by performing genetic experiments in which mutant pRB and p107 proteins will be re-introduced into various tumor cell lines with an activated Ras allele to determine which region(s) of these proteins confers their distinct biological properties in this context. In addition, it is important to determine how Ras activation as well as other known RB regulatory molecules modulates pRB expression and activity in human tumor cells in which this tumor suppressor show activities reminiscent of an oncogene. The intriguing observation regarding the antagonistic relationship between Ras activation and pRB-loss in human tumor cells will also be extended to include a large number cell lines from different tumor types. Together, these studies are expected to provide significant clues regarding the mechanism for the observed functional differences among these closely related proteins in tumorogenesis.